kernel: use stow_copy_files for /lib/modules/<version>/build.

srcpkgs/kernel/template

@@ -18,7 +18,12 @@ triggers="initramfs-tools"
 nostrip=yes
 noverifyrdeps=yes
 preserve=yes
-
+if [ -n "${revision}" ]; then
+	_kernver="${version}_${revision}"
+else
+	_kernver="${version}"
+fi
+stow_copy_files="/lib/modules/${_kernver}/build"
 Add_dependency run glibc
 Add_dependency full coreutils
 Add_dependency full module-init-tools